terminology: #[feature] *activates* a feature (instead of "declaring" it)

[RalfJung]

Mostly, we currently call a feature that has a corresponding #[feature(name)] attribute in the current crate a "declared" feature. I think that is confusing as it does not align with what "declaring" usually means. Furthermore, we also refer to #[stable]/#[unstable] as declaring a feature (e.g. in these diagnostics), which aligns better with what "declaring" usually means. To make things worse, the functions tcx.features().active(...) and tcx.features().declared(...) both exist and they are doing almost the same thing (testing whether a corresponding #[feature(name)] exists) except that active would ICE if the feature is not an unstable lang feature.

So really, our terminology is just a mess.

I would suggest we use "declaring a feature" for saying that something is/was guarded by a feature (e.g. #[stable]/#[unstable]), and "activating a feature" for #[feature(name)]. This PR implements that.

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